Starting with dev-vcs/git-2.26.0, running `git rebase` against a branch that has no commits but is behind its upstream results in "error: nothing to do" if `rebase.abbreviateCommands` is enabled. Upstream bug report at: https://lore.kernel.org/git/9b4bc756764d87c9f34c11e6ec2fc6482f531805.camel@gmail.com/
This issue was fixed in dev-vcs/git-2.27.0 (but still exists in dev-vcs/git-2.26.2).
dev-vcs/git-2.32.0-r1 is the oldest version in the tree now. Closing.